1. Field of Invention
Various embodiments of the present invention relate generally to a semiconductor memory device and, more particularly, to a semiconductor memory device having an improved data retention characteristic.
2. Description of Related Art
A NAND flash memory device is a type of non-volatile semiconductor memory devices. In a NAND flash memory device, the threshold voltage of a memory cell is shifted and the data stored in each memory cell is determined based on the amount of electrons injected into the floating gate of the memory cell. In order to retain the data stored in a memory cell, the electrons injected into the floating gate need to be minimized.
With increasing degree of high integration for larger data storage capacity, the intervals between the memory cells are narrowed, which leads to increased parasitic capacitance between the memory cells. Then, when a high voltage is applied to a memory cell, the high voltage is also transferred to the adjacent memory cells because of the capacitor coupling phenomenon, and the electrons are discharged from the floating gate of the memory cell due to the high voltage.
Particularly, during the data input and output (I/O) operations, the data stored in the unselected memory cells may be altered depending on the voltage applied to a bulk (e.g., a P well) in which the bit lines are placed over the memory cells because the electrons stored in the floating gates of the unselected memory cells are discharged when the voltage is applied to the unselected memory cells.